Cleanup electrode



Nov. 26, 1968 ROBNSON 3,412,710

CLEANUP ELECTRODE Filed Oct. 11, 1966 INVENT BRUCE RROBI ON QQQ .AT olefin s United States Patent 3,412,710 CLEANUP ELECTRODE Bruce R. Robinson, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Oct. 11, 1966, Ser. No. 585,921 3 Claims. (Cl. 118-637) ABSTRACT OF THE DISCLOSURE A high potential electrode for minimizing uncontrolled powder cloud development in a xerographic two-component cascade development system. The electrode is positioned adjacent the space where cascading developer falls away from th photoconductive surface under the influence of gravity. The electrode has a shape to substantially conform with the path of flow of the falling carrier granules. It is electrically biased so as to attract free toner particles back into the path of fiow of carrier granules falling into the sump for minimizing powder cloud development in this area.

This invention relates in general to xerography and in particular to apparatus for use in two-component cas cade development systems.

In the practice of xerography, as described for example in US. Patent No. 2,297,691, to Chester F. Carlson, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process, the xerographic plate is electrostatically charged uniformly over its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern.

The latent electrostatic image can then be developed by contacting it with a finely divided electrostatically attractable material such as a powder. The powder is held in image areas by the electrostatic charge on the layer. Where the charge is greatest, the greatest amount of material is deposited; and where the charge is least, little or no material is deposited. Thus, a powder image is produced in conformity with the light image of the copy being reproduced. The powder is subsequently transferred to a sheet of paper or other surface and suitably aflixed thereto to form a permanent print.

The electrostatically attractable developing material commonly used in xerography consists of a pigmented resinous powder referred to herein as toner and a coarse granular material called carrier. The carrier is usually a glass, sand or steel bead coated with and encased in a material removed in the triboelectric series from the toner so that a triboelectric charge is generated between the powder and the granular carrier upon mutual interaction. Such charge causes the toner powder to adhere to the carrier. When the toner latent carrier is moved into contact with a latent electrostatic image, the charge of the image areas attracts the toner from the carrier to the image areas to thus render the latent electrostatic image visible.

Many development systems have been proposed for bringing the charged toner into contact with the image bearing surface. One of such systems in wide use is the cascade development system. In cascade development the two component developer is poured or cascaded across a xerographic surface under the action of gravity. The transfer of toner to the image bearing surface, in theory at least, is achieved by the electrostatic attraction of the Patented Nov. 26, 1968 "ice image areas for the toner particles overcoming the electrostatic attraction of the carrier for the toner. In practice, however, appreciable quantities of toner particles are jarred loose fom their associated carrier granules when dropped onto, or while cascading across, the surface being developed. Such unassociated toner particles accumulate into powder clouds and deposit themselves randomly on xerographic surfaces. The toner particles of the clouds often attach themselves to portions of the xerographic surface representing non-image areas, which if left there and transferred to the final copy sheet would exhibit themselves as unwanted background. Background can also occur from toner particles attaching themselves to non-image areas merely by the mechanical attraction between toner and non-image portions of the surface.

Not all toner particles which come to rest on nonimage areas remain there for transfer to the final copy. A great percent of such toner is removed by the scavenging effect of the carrier granules. Scavenging occurs when carrier particles move across a toner bearing surface after depositing their previously associated toner particles onto image areas. As the carrier granules continue their movement across the surface, toner to carrier contact can attract those toner particles from the surface which are held thereto by weak electrostatic forces. Such forces are characteristic of the forces holding toner particles to non-image areas. Consequently, scavenging carrier granules tend to remove toner particles which has been undesirably deposited in non-image areas.

In the usual practice of cascade development, as practiced on continuous and automatic machinery wherein the xerographic surface is formed as a rotary drum, belt or the like, the developer is dropped onto the xerographic surface somewhere adjacent the upper portion of the surface. The developer then cascades down the drum until it arrives at approximately the horizontal center line of the drum at which time it drops from the drum under the influence of gravity. The internal turbulence of the cascading developer, as mentioned above, gives rise to an agitated cloud of freely floating toner along its path which deposits toner onto all areas of the Xerographic surface including the area beneath the line where the carrier has fallen from the drum. Since the carrier has gravity fallen from the drum surface at the area adjacent the horizontal center line, there is no chance to remove the background toner from this part of the drum by scavenging therebetween. It has thus been the practice to retain this background toner on the drum. Attempts have been made to minimize the background created by this cloud by closely regulating the toner concentration viz., the percentage of toner to carrier.

The present invention is directed to a novel approach for eliminating background as caused by uncontrolled powder clouds in the area where the developer gravity drops from the xerographic surface by minimizing the clouds electrostatically. This is achieved through a minimum of structure which is readily incorporated into existing xerographic machinery.

It is therefore an object of the present invention to develop latent electrostatic images.

It is a further object of the present invention to improve two component cascade development systems.

It is still a further object of the present invention to reduce background in two component development systems by cloud suppressing electrodes.

It is still a further object of the present invention to minimize unwanted powder cloud development in twocomponent cascade development systems.

It is a further object of the invention to decrease the amount of powder cloud toner seeping out of xerographic development stations past the pick off baflle.

It is a further object of the invention to control powder cloud development on xerographic surfaces in the region where developer gravity drops from the surface.

It is still a further object of the invention to electrostatically remove background depositing clouds of toner from a xerographic surface during development.

These and other objects of thepresent invention are achieved by a cloud suppressing electrode positioned adjacent the area where cascading two-component developer material drops away from a xerographic surface being developed. By biasing the electrode to a high potential and to a polarity opposite from that on the toner, toner clouds generated in this area may be drawn away from the xerographic surface for minimizing the background which would otherwise occur.

For a better understanding of the invention, as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in conjunction with the accompanying drawings, wherein;

FIG. 1 is a diagrammatic side across sectional view of a xerographic machine adapted for continuous and automatic operation embodying the principles of the instant invention;

FIG. 2 is a diagrammatic side cross sectional view of the development instrumentalities shown in FIG. 1 but enlarged for greater clarity;

FIG. 3 is a diagrammatic side cross section view of a second embodiment of development instrumentalities constructed in accordance with the instant invention.

Shown in FIGURE 1 is a xerographic machine constructed for continuous and automatic operation and embodying the principles of the instant invention. All of the processing stations referred to by letters are conventional in the xerographic art except for the development station C which forms the basis of the instant invention. For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the xerographic drum may be described as follows:

A charging station A, at which a uniform electrostatic charge is deposited on the photoconductive layer of the xerographic drum;

An exposure station B, at which the light or radiation pattern of copy to be reproduced is projected onto the drum surface to dissipate the charge in the exposed areas thereof to thereby form a latent electrostatic image of the copy to be reproduced;

A development station C, at which a xerographic developing material including toner powder having an electrostatic charge opposite to that of the latent electrostatic image, is moved into contact with the drum surface, whereby the toner powder adheres to the latent electrostatic image to form a xerographic powdered image in the configuration of the copy being reproduced;

A transfer station D, at which the xerographic powdered image is electrostatically transferred from the drum surface to a transfer material or a support surface;

A drum cleaning station E, at which the drum surface is brushed to remove residual toner particles remaining thereon after image transfer.

As stated above, all of these stations are conventional in the xerographic art except for the development station C.

The developing instrumentalities are positioned adjacent the xerographic surface 10 upon which the images are adapted to be formed. The xerographic surface is shown in the illustrated embodiment as a rotating drum movable to sequentially move its surface portions in a path through the various processing stations for an automatic and continuous cycle of operation. Power may be applied for rotating the drum, as well as for initiating the cycle of operation for the various processing stations by any conventional power source, not shown. A xerographic surface with any cylindrical cross section could be used. The cross section need not be circular.

The developing instrumentalities are housed within a general developer housing 12. The lower or sump portion of the developer housing is adapted to be filled with a quantity of two component developer material. The developer may be raised to an elevated position for cascading down the xerographic surface by a series of buckets 14 movable on a belt 16 and guided for its motion by rollers 18. Power may be imparted to the rollers by any conventional power source, not shown, to move the buckets in the direction as indicated by the arrows.

As the buckets reach their uppermost position they are adapted to drop the developer through a pair of plates 20, 22 for guiding the developer onto the drum surface. The sump, buckets, and plates extend a length approximately equal to the length of the drum to insure the cascading of developer across the length of the drum. As the developer cascades down the arc of the drum it develops the latent electrostatic images on the drum surface which it contacts. As the developer falls past the horizontal center line of the drum its gravity drops onto the pick off baflie 24 and back into the sump for recycling through the development zone. A toner dispenser may be incorporated into the developer housing for supplementing the toner given up by the system through development of images.

When cascading down a curved sector of the drum surface, the carrier granules follow a path defined by the curvature of the drum. They then fall away from the drum under the influence of gravity. Since the carrier granules have a component of motion in the horizontal direction, they leave the drum surface slightly above the horizontal center line of the drum due to the combination of gravity and inertia action on the moving carrier granules. Their high mass prevents them from being stirred up into uncontrolled clouds. This is not so of the toner particles which are smaller in size and specific gravity than the carrier granules. The toner particles of the developer are agitated during cascading into randomly moving clouds which follow the movement of the carrier granules in only a general fashion, extending outside the normal flow path of the carrier granules. Thus the movement of the toner clouds is over an extended path which contacts the drum in a toner depositing manner including an area beneath the line where the carrier granules drop away from the drum.

Positioned adjacent the drum and preferably concentric therewith is a development electrode 26 and a cloud suppressing electrode 28. So that a bias may be placed on each of these electrodes independent of the influence of the other, an insulating strip 30 is positioned therebetween. An individual bias may be applied to the two electrodes by potential sources 32 and 34 respectively. An insulating strip 30, constructed, as for example, of a butyl rubber or the like, permits these potentials to be applied independent of each other.

The development electrode 26 is constructed of a conductive material, aluminum for example, and positioned closely spaced from the xerographic surface being developed. This electrode may be biased, as by potential source 32 to a polarity the same as that on the image and non-image areas of the xerographic surface for conventional development. This would be of a polarity opposite from the toner particles. The potential on the electrode should be of a slightly greater magnitude than that in background areas of the latent electrostatic image for attracting toner away from such non-image areas to minimize background.

The development electrode may also be grounded with respect to the substrate of the xerographic surface in which case its effect would not be on non-image areas of the latent electrostatic image but would rather strengthen large area portions of the images. Development electrodes are discussed generally in Ulrich Patent No. 3,011,474, Clark et al. Patent No. 2,952,241 and Landrigan et al. Patent No. 2,752,304.

Positioned beneath the development electrode 26 is the cloud suppressing electrode 28 which is the basis of the instant invention. This electrode as shown in the figures is adapted to be positioned adjacent the horizontal center line of the xerographic drum. When this electrode is biased to a high potential and of an opposite polarity as the toner particles it attracts toner clouds which are normally formed by the cascading developer in this region away from the xerographic drum. Since this electrode is adapted to be biased, as by a potential source 34, it is preferably constructed of a conductive material, aluminum for example.

Under the normal influence of gravity, developer tends to leave the drum surface slightly above the horizontal center line of the drum. This is due to the horizontal component of its inertia which is generated as the developer falls down the surface of the drum. No scavenging of background occurs beneath a line slightly above the horizontal center line of the drum surface since no carrier contacts the drum therebeneath. As such, the cloud suppressing electrode is preferably located to span the space both above and below this horizontal center line. The electrode is preferably angularly positioned with respect to the vertical, substantially parallel with the path of flow of the carrier to contact only the edge of the flow, so as not to severely deflect the tangentially moving developer back onto the drum beneath the center line. The specific angle of the electrode is a function of many variables such as the speed and size of the developer flow and the diameter and are of the drum employed. In practice, some carrier is deflected onto the pick off baffie. The cloud suppressing electrode 28 acts due to its potential to electrostatically draw the uncontrolled powder cloud away from the image in this area back into the normal flow taken by the heavier carrier particles which by the nature of their specific gravity and speed do not form such uncontrolled clouds. The lowermost portions of the electrode 26 are at increasing distances away from an imaginary vertical plane between the photoconductive surface and the electrode. It is preferable that the distance between the photoconductive surface and the electrode be greater adjacent the lower end of the electrode than adjacent the upper end thereof so that the profile of the electrode approximates the path of flow of carrier falling from the photoconductive surface.

The opposite polarity between the toner and cloud suppressing electrode 28 causes toner to collect on the electrode. By positioning the electrode so that it lies adjacent the normal tangential path of the carrier portion of the developer, the carrier will in fact triboelectrically reattract toner from the electrode for future recirculation. In practice, a slightly toner coated electrode 28 has been found to render no ill effects to the operation of the invention.

Shown in FIGURE 3 is another embodiment of the instant invention, similar to that as described with respect to FIGURES 1 and 2. In this embodiment the development electrode 36 is adapted to be mounted in position adjacent the image bearing surface by a circuit board 38 constructed by an insulating material as for example, a phenolic resin or the like. The cloud suppressing electrode 40 is constructed of a thin sheet of a conductive material such as copper or the like, suitably bonded to the circuit board 38. The development electrode 36 and cloud suppressing electrode 40 are preferably spaced from each other by the circuit board or any other suitable insulator for the application of independent potentials. The potentials may be applied by sources of potential 42 and 44 respectively.

In this embodiment, the pick off bafile is modified over that as described with respect to the first embodiment. In the first embodiment the pick off bafiie 24 was formed as an extension of the developer housing 12. In normal practices this pick off baffie is constructed of rigid metallic material. This is substantially analogous to the pick off baftie 46 of FIGURE 3. The modified pick off baffle 46 of FIGURE 3, however, is constructed with a conductive sheet 48 secured to its upper surface adjacent its end closest to the drum. It is secured in place through an insulating layer 50 by any convenient cement like material. The insulating layer 50 permits the application of a bias to the plate 48 from a potential source 52 without interference from the conductive lower plate 46. This conductive plate 48 is adapted to be biased to a high potential and of an opposite polarity from the toner particles to attract thereto any remnants of a powder cloud which may have fallen past the effective field of the cloud suppressing electrode 40. This tends to extend the effective field of the cloud suppressing electrode 40. Carrier which may fall across this plate 48 tends to clean off toner which has been attracted to this plate. As described with respect to the cloud suppressing electrode, a constant but controlled build up of toner on this biased element 48 has not been found to deleteriously affect its operation.

In operation, as the cascading two-component developer tumbles down the surface of a xerographic drum, a cloud of the lighter mass toner is created along the path of fiow of the developer. The cloud constantly deposits toner over both image and non-image areas of the xerographic surface. Subsequent carrier particles which have given up some of their toner act to scavenge toner deposits from non-image areas during their course of movement down the surface. Acting under the influence of gravity and inertia, a stream of the heavier carrier particles leaves the curved xerographic surface slightly above the horizontal center line of the drum. At this time, the edge of the stream of carrier contacts the face of the cloud suppressing electrode. A slight amount of carrier is also deflected onto the pick off bafiie. The toner clouds which are formed would normally deposit toner on the drum in the area slightly above and below the horizontal center 1ine of the drum. Due to the absence of scavenging carrier contacting the drum in this region such clouds would normally leave unwanted background in non-image areas of the latent electrostatic image being developed. Due to the presence of the clean up electrode 28 or 40 as well as the biased pick off plate 48 employed in the embodiment of FIGURE 3, these clouds are drawn away from the xerographic surface back into the normal flow of the carrier for minimizing the deleterious background which would otherwise occur. The attraction of the toner cloud away from the xerographic drum has the further advantage of minimizing a portion of such toner clouds which would otherwise seep past the space between the drum and pick off batlie. The movement of a toner cloud beyond the pick off baflie causes toner to be lost from the development system and also causes toner deposits throughout the other functioning elements of the machinery.

As applied to conventional xerographic machinery employing negative toner, a positive volt bias on-the development electrode 26 or 36, a positive 1500 volt bias on the cloud suppressing electrode 28 or 40 and a positive 500 volt bias on pick off plate 48 has been found to offer much improved results in development. In practice, however, it should be understood, that the clean up electrode may be employed with any type of biased or grounded development electrode or even in the absence of a development electrode.

While it has been disclosed that the instant invention is useable in commercial machines employing positive charging of the xerographic surface, negative toner and positive electrodes it should be understood that the selection of such polarities has been done merely for illustrative purposes. In like manner, the recited potentials and polarities of the various components have also been selected for illustrative purposes since the instant invention is operable through a wide range of parameters.

While the present invention, has its objects and adr vantages, has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby; but it is intended to cover the invention broadly within the scope of the appended claims.

What is claimed is:

1. Apparatus for minimizing uncontrolled powder cloud development in xerographic development apparatus of the type wherein latent electrostatic images on an endless photoconductive surface are developed with a twocomponent developer including charged toner particles and carrier granules cascaded across the photoconductive surface creating powder clouds of charged toner particles along its path as well as adjacent a line where the developer gravity drops from the surface comprising a conductive plate,

means to secure the plate in spaced registration from the photoconductive surface adjacent the line where the cascading developer drops from the photoconductor surface with at least the major extent of the conductive plate being separated from the photoconductive surface by an imaginary vertical plane between the conductive plate and the photoconductive surface with the lowermost portions of the conductive plate being spaced farther from the imaginary vertical plane than the portions thereabove and means to electrically bias the conductive plate to a polarity opposite from that on the charged toner particles for attracting the toner clouds away from the photoconductive surface.

2. The apparatus as set forth in claim 1 wherein the distance between the conductive plate and the imaginary vertical plane gradually decreases from the lower end of the conductive plate towards the upper end of the conductive plate.

3. Apparatus for minimizing uncontrolled powder cloud development in xerographic reproducing apparatus 35 of the type wherein latent electrostatic images on a drum shaped photoconductive surface are developed with a twocomponent developer comprising charged toner particles and carrier granules cascaded across the surface creating powder clouds of charged toner particles along its path as well as adjacent a line where the developer gravity drops from the surface, the improvement comprising a conductive plate, means to secure the plate in spaced registration from the surface adjacent the line where the cascading developer gravity drops from the surface with the upper edge of the plate above the horizontal centerline of the surface and the lower edge of the plate beneath the horizontal centerline of the surface with the conductive plate being substantially parallel to the flow of the developer as it drops from the surface, the conductive plate being located a distance from the surface whereby a portion of the flowing developer can contact the conductive plate, means to electrically bias the conductive plate to a polarity opposite from that on the charged toner particles for attracting the clouds away from the surface, a developer pick off bafile positioned beneath the conductive plate to catch the falling developer and means to electrically bias the pick off bafiie to a polarity opposite from that on the charged toner particles for attracting the clouds away from the surface.

References Cited UNITED STATES PATENTS 2,573,881 11/1951 Walkup et al. 118-637 XR 2,952,241 9/1960 Clark et al. 118-637 3,011,474 12/1961 Ulrich 118-637 3,147,147 9/1964 Carlson 118-637 3,303,817 2/1967 Cranch et al 118637 PETER FELDMAN, Primary Examiner. 

